The present invention relates to electric motors and in particular to permanent magnet synchronous machines (PMSM) and more particularly to a method and apparatus for determining the starting position of the rotor of a PMSM without a position sensor such as a resolver.
Permanent magnet synchronous machines are electric motors having a rotor holding a permanent magnet that may turn about an axis within a stator. The stator holds conductive coils that may be energized to create a rotating magnetic field. The rotating magnetic field is coordinated with the rotor position to draw the rotor along synchronously.
Simple PMSM's employ power transistors to switch the electrical currents in the stator coils to create the necessary rotating magnetic field. “Sensor-less” PMSM's eliminate the rotor position sensors and deduce rotor position from its effect on the electrical signals used to drive the stator coils.
In these sensor-less PMSM's, rotor position may be simply determined while the motor is operating (and the rotor is spinning) by means of the electrical voltages generated (induced) by the rotating magnetic rotor in the stator windings (so called “back-EMF”). Unfortunately, when the rotor is moving at a low speed or stationary, the back-EMF is low or nonexistent making it difficult to determine rotor position. Starting a PMSM motor without knowing the rotor position causes sudden accelerations of the rotor (possibly in the wrong direction) as the rotor attempts to align itself with the generated field. In many important motor control applications, such abrupt and unpredictable motion is undesirable.
To overcome this problem, an approach has been developed for identifying rotor position that does not rely on back-EMF and thus that can work for a stationary rotor. This approach relies on variations in magnetic saliency of the rotor. Magnetic saliency refers to a change in the inductance of the stator windings as a function of the orientation of the rotor and results generally from the anisotropic magnetic properties of the rotor.
A typical approach to identifying rotor position using magnetic saliency is described in U.S. Pat. No. 6,172,498 in which pulses are applied to each of the stator windings in sequence and variations in measured saliency is used to deduce the approximate location of the rotor.
One limitation to this approach is that special hardware may be required.